Steel for Hot Tooling, and Part Produced From Said Steel, Method for the Production Thereof, and Uses of the Same

ABSTRACT

The invention relates to steel for hot tooling, the composition of said steel being made up of the following weight percentages: 0.30%=C=0.39%, 4.00%=Cr=6.00%, traces=Si=0.50%, traces=Mn=0.80%, traces=W=1.45%, traces=Co=2.75%, 0.80%=Ni=2.80%, 1.50%=Mo=2.60% with 1.50%=Mo+0.65W=3.20%, 0.55%=V=0.80%, with 0.65=K=0.65, where K=K2−K1 and K2=0.75×(Ni 0.60), K1=1.43×(V0.40)+0.63×[(Mo+0.65W) 1.20], traces=Al=0.080%, traces=S=0.0040%, traces=P=0.0200%, traces=Ti=0.05%, traces=Zr=0.05%, traces=Nb=0.08%, traces=N=0.040%, 10P+As+5Sb+4Sn=0.21%, traces=O=30 ppm, the remainder being iron and inevitable impurities. The invention also relates to a part produced from said steel, to the method for the production thereof, and to the use of the same.

The invention relates to the field of steels for tooling for hot shaping, which can be used in foundry and moulding, forging, drawing or extrusion.

A field of application of the invention which is preferred but not exclusive is the production of moulds of large dimensions for foundry under pressure of light alloys based on aluminium or magnesium or of cuprous alloys.

During use, tools for hot shaping are subjected to cyclical stresses which damage them.

The origins of these stresses are:

-   -   mechanical due to direct forces applied by the machines such as         presses;     -   thermal: sudden variations in temperature due to alternating         contacts with the hot material to be transformed and to cooling         by the splashing of lubricants or of refractory washes, cause         expansion gradients which are sources of local mechanical         stresses.

The damage is produced in certain cases by sudden ruptures which instantaneously destroy the tool when the toughness of its material is insufficient. It is generally produced by cracking which starts during the first few hundred cycles of use and develops progressively until the tool is effectively ruined after several tens or hundreds of thousands of cycles. This process is designated by the generic term of “thermal fatigue”.

Resistance to damage by thermal fatigue requires a toughness which is sufficient at the temperature corresponding to the coolest point in the thermal cycle. This quality is conventionally measured by the energy of flexion due to shock of standard testpieces, testpieces tested at temperatures between ambient temperature and 150° C. It also requires sufficient properties of hardness and of resistance to softening in use at the hottest temperatures of the cycle.

The manufacture of moulds or tools of a substantial size (for example having a thickness greater than 200 mm) demands even further improved properties of the steel from which they are made. During quenching, as the rate of cooling is naturally moderated by thermal flows limited to the surfaces and the concern of the manufacturer not to deform or break the parts, the steels in question do not generate predominantly martensitic quenching structures which would favour optimal properties for use. The QCC diagrams (quenching by continuous cooling) describe for each composition the nature of the phases formed according to the rates of cooling, but they are well known to be insufficient for giving an account of the loss of toughness in the quenched/tempered state caused by the reduction in the rate of quenching.

Amongst the steels which are known for this use mention may be made of:

-   -   AISI H11 steel which contains approximately C=0.40%, Si=0.90%,         Mn=0.40%, Cr=5%, Mo=1.30%, V=0.5%;     -   AISI H13 steel identical to the previous one, except that it         contains V=0.95%;     -   W-1.2367 steel which contains approximately C=0.40%, Si=0.30%,         Mn=0.40%, Cr=5%, Mo=2.9%, V=0.65%;     -   a steel which is comparable to AISI H11 but contains Si=0.3% and         accepts Ni=0.2% (see the document EP-B1-0 663 018); its nominal         composition is C=0.3-0.4%, Si≦0.8%, Mn≦0.8%, Cr=4.5-5.8%,         Mo=0.75-1.75%, V≦1.3%, W≦1.5%, Ni≦0.5%, P≦0.008%, Sb≦0.002%,         Sn≦0.003%, As≦0.005%, with 10P+5Sb+4Sn+As≦0.10%.

In order to improve the properties of these known steels, studies have been carried out with a view to obtaining a better compromise between hardness, toughness and stability of the properties in use, particularly the hardness. Thus it has been possible to raise the resistance when hot relative to H11 steel by increasing the contents of Mo and V as in the H13 and W-1.2367 steels referred to above, but this results in a decrease in the toughness. On the other hand, the toughness is increased if the Si content is reduced or if Ni is added, which also increases the quenchability. However, the Ni decreases the hardness and the yield strength when hot.

The object of the invention is to propose a novel grade of steel for hot shaping tools producing an excellent compromise between the various properties which have just been mentioned.

To this end the invention relates to a steel for hot tooling, having a composition in percentages by weight:

-   -   0.30%≦C≦0.39%     -   4.00%≦Cr≦6.00%     -   traces≦Si≦0.50%     -   traces≦Mn≦0.80%     -   traces≦W≦1.45%     -   traces≦Co≦2.75%     -   0.80%≦Ni≦2.80%     -   1.50%≦Mo≦2.60% with 1.50%≦Mo+0.65W≦3.20%     -   0.55%≦V≦0.80%     -   with −0.65≦K≦0.65

where K=K2−K1

and K2=0.75×(Ni−0.60)

K1=1.43×(V−0.40)+0.63×[(Mo+0.65W)−1.20]

-   -   traces≦Al≦0.080%     -   traces≦S≦0.0040%     -   traces≦P≦0.0200%     -   traces≦Ti≦0.05%     -   traces≦Zr≦0.05%     -   traces≦Nb≦0.08%     -   traces≦N≦0.040%

10P+As+5Sb+4Sn≦0.21%

-   -   traces≦O≦30 ppm

the rest being iron and inevitable impurities.

Preferably 0.33%≦C≦0.38%.

Preferably, traces≦Si≦040%.

Preferably, traces≦Mn≦0.60%.

Preferably, 4.6%≦Cr≦6.0%.

Preferably, 1.60%≦Mo≦2.00% and 1.60%≦Mo+0.65 W≦2.20%.

Preferably, traces≦Al≦0.030%.

Preferably, traces≦S≦0.0010%.

Preferably, traces≦P≦0.0080%.

Preferably, traces≦Ti≦0.01%.

Preferably, traces≦Zr≦0.02%.

Preferably, traces≦Nb≦0.01%.

Preferably, traces≦N≦0.01%.

Preferably, 10 P+As+5Sb+4Sn≦0.10%.

Preferably, traces≦O≦15 ppm.

Preferably, −0.35≦K≦0.35.

Preferably:

-   -   0.335%≦C≦0.375%     -   1.50%≦Ni≦2.10%     -   1.60%≦Mo+0.65 W≦2.20% with 1.60%≦Mo≦2.00%     -   0.62%≦V≦0.75%.

Preferably:

-   -   0.335%≦C≦0.375%     -   2.00%≦Ni≦2.40%     -   1.80%≦Mo+0.65W≦2.90% with 1.80%≦Mo≦3.40% and W≦0.90%     -   0.66%≦V≦0.76%.

Preferably:

-   -   0.335%≦C≦0.375%     -   0.90%≦Ni≦1.50%     -   1.50%≦Mo+0.6 W≦1.90% with W≦0.40%     -   0.55%≦V≦0.63%.

Preferably:

0.335%≦C≦0.375%, 4.60%≦Cr≦6.00%, traces≦Si≦0.40%, traces≦Mn≦0.60%, traces≦W≦1.45%, traces≦Co≦2.75%, 1.50%≦Ni≦2.10%, 1.60%≦Mo+0.65 W≦2.20% with 1.60%≦Mo≦2.00%, 0.62%≦V≦0.75%, with −0.35≦K≦0.35, traces≦Al≦0.030%, traces≦S≦0.0010%, traces≦P≦0.0080%, traces≦Ti≦0.011%, traces≦Zr≦0.02%, traces≦Nb≦0.01%, traces≦N≦0.01%, traces≦O≦15 ppm.

The invention also relates to a method of manufacture of a part made from steel, characterised in that the said steel is prepared from a steel of the preceding type and in that it is subjected to austenisation in the temperature range from 1000 to 1050° C., followed by quenching.

Preferably, the austenisation takes place in the range from 1015 to 1040° C.

Preferably, after the quenching the part is subjected to at least two temperings in the temperature range from 550 to 650° C., giving the said part a hardness of 42 to 52 HRC.

The invention also relates to a part made from steel obtained by the preceding method, characterised in that it is a part for tooling for hot shaping.

The said part can have a thickness greater than or equal to 200 mm.

It may be a mould or a die for foundry under pressure of light or cuprous alloys.

The said part may be a forging tool.

The said part may be a forging die.

The said part may be a tool for drilling or rolling steel tubes.

The said part may be a tool for shaping glass.

The said part may be a tool for shaping plastics materials.

The said part may be produced from a steel in which 0.335%≦C≦0.375%, 2.00%≦Ni≦2.40%, 1.80%≦Mo+0.65W≦2.90% with 1.80%≦Mo≦3.40% and W≦0.90%, 0.66%≦V≦0.76%, and it is an extrusion die or a mould for founding aluminium alloy.

The invention also relates to use of a part for hot tooling, characterised in that the said part is made from a steel in which 0335%≦C≦0375%, 2.00%≦Ni≦2.40%, 1.80%≦Mo+0.65W≦2.90% with 1.80%≦Mo≦3.40% and W≦0.90%, 0.66%≦V≦0.76% and its working temperature at the surface remains lower than 680° C.

The invention also relates to use of a part for hot tooling, characterised in that the said part is made from a steel in which 0.335%≦C≦0.375%, 0.90%≦Ni≦1.50%, 1.50%≦Mo+0.6 W≦1.90% with W≦0.40%, 0.55%≦V≦0.63% and its surface temperature in use remains lower than 770° C.

As will be understood, by comparison with the known steels which were previously mentioned, in particular the steel described in EP-B1-0 663 018, the invention is based in particular on a simultaneous adaptation of the softening and stabilising elements which are Mo and V, and Ni which neutralises their weakening effects. The joining of the whole produces an improvement in the quenchability and therefore improves the capacity for reproducing on large parts the properties which until then had only been available on smaller tools.

The optimisation according to the invention of the composition of the steel has been possible because the inventors initially devoted themselves to effective measurement of the instantaneous heat flows which pass through the surface of the hot shaping tools while they are in use. They are then deduced therefrom by calculation of the transitory mechanical stresses induced by the thermal shocks which cause the cracks. This has made possible a better understanding of the mechanical behaviour of the material in operation. They were able to establish, by virtue of the experimental measurements which reconstitute the industrial quenching speeds on test samples, and by virtue of the thermodynamic simulations, the links which exist between the composition of the steel, the parameters of the heat treatment prior to its implementation and the microstructure thus obtained. In particular they demonstrated the crucial importance of the interdependence between the composition and the quenching temperature for obtaining the compromise sought between the various mechanical properties which are important in steels for hot tooling.

The invention will be better understood by reading the following description which is given with reference to the appended drawings:

FIG. 1 which shows the evolution of the fraction of undissolved carbides according to the permitted temperature for the reference compositions (FIGS. 1 a) to 1 e)) to produce a composition according to the invention (FIG. 1 f)),

FIG. 2 which shows the QCC curves of a reference steel (FIG. 2 a)) and of a steel according to the invention (FIG. 2 b)).

FIG. 3 which shows the comparison, for various reference samples and samples according to the invention, between the breaking energies after quenching carried out under laboratory conditions and quenching carried out under industrial conditions.

The tests to which reference will be made in the following description were carried out on samples of which the compositions are set out in Table 1. In this table the coefficients K2, K1 and K correspond to the following quantities where the contents are expressed in % by weight:

TABLE 1 Compositions of the test samples. Nature of Casting casting reference C % Si % Mn % Ni % Cr % Mo % V % W % Co % REFERENCES 1 0.35 0.28 0.35 0.06 5.11 1.21 0.47 0.01 0.00 2 0.36 0.32 0.30 0.11 5.02 1.27 0.49 0.03 0.05 3 0.36 0.21 0.37 0.16 5.19 1.74 0.49 0.01 0.05 4 0.33 0.28 0.36 0.09 5.10 1.83 0.46 0.01 0.00 5 0.38 0.31 0.30 0.17 5.04 2.74 0.48 0.02 0.02 6 0.36 0.27 0.38 0.11 5.05 2.24 0.54 0.01 0.06 7 0.36 0.15 0.51 0.08 5.17 2.29 0.57 0.01 0.01 8 0.36 0.32 0.42 0.15 4.98 1.62 0.64 0.02 0.01 9 0.35 0.29 0.43 0.25 5.01 1.59 0.66 0.02 0.01 10 0.34 029 0.35 0.02 5.11 1.23 0.84 0.01 0.00 11 0.35 0.17 0.35 0.33 5.00 1.13 0.69 0.01 0.05 12 0.35 0.28 0.35 1.42 5.13 1.21 0.46 0.01 0.00 13 0.36 0.30 0.34 2.93 5.27 1.23 0.47 0.01 0.01 14 0.33 0.28 0.35 1.83 5.18 1.83 0.47 0.01 0.00 15 0.36 0.29 0.36 1.82 5.19 1.22 0.72 0.01 0.00 16 0.41 0.21 0.38 1.62 5.21 1.76 0.69 0.01 0.04 17 0.35 0.27 0.35 0.07 5.24 1.23 0.47 0.01 2.72 18 0.35 0.20 0.33 0.06 5.05 1.22 0.45 1.21 0.01 19 0.36 0.29 0.36 1.85 5.25 1.23 0.47 1.65 0.01 20 0.34 0.18 0.61 0.59 5.17 2.14 0.77 0.03 0.56 INVENTION 21 0.37 0.32 0.41 1.64 4.95 1.76 0.67 0.02 0.01 22 0.35 0.25 0.44 1.63 5.03 1.82 0.71 0.01 0.01 23 0.35 0.24 0.34 1.05 5.15 1.78 0.70 0.01 0.01 24 0.37 0.26 0.36 1.68 5.05 2.26 0.71 0.01 0.01 25 0.37 0.31 0.41 2.20 4.98 1.79 0.71 0.02 0.01 26 0.37 0.31 0.40 2.19 4.99 2.28 0.70 0.02 0.01 27 0.35 0.32 0.42 1.66 5.12 1.83 0.66 0.03 2.07 Nature of Casting S P Al O As Sb Sn casting reference ppm ppm ppm ppm ppm ppm ppm K2 K1 K REFERENCES 1 10 40 165 12 32 8 35 −0.41 0.11 −0.52 2 8 53 220 11 27 11 42 −0.37 0.18 −0.55 3 17 44 145 14 18 13 32 −0.33 0.47 −0.80 4 11 57 175 10 16 6 12 −0.38 0.49 −0.87 5 <1 130 245 13 69 7 42 −0.32 1.09 −1.42 6 28 44 150 8 22 9 39 −0.37 0.86 −1.23 7 3 93 295 13 55 12 43 −0.39 0.93 −1.32 8 7 52 290 11 23 11 38 −0.34 0.62 −0.95 9 11 50 55 9 20 4 32 −0.26 0.63 −0.89 10 11 47 260 7 7 14 8 −0.44 0.65 −1.09 11 15 37 255 12 15 11 35 −0.20 0.37 −0.58 12 9 37 180 8 8 13 15 0.62 0.10 0.52 13 10 36 140 9 12 12 9 1.75 0.12 1.62 14 10 39 120 10 16 9 14 0.92 0.50 0.42 15 10 40 115 10 12 14 13 0.92 0.47 0.44 16 19 47 150 10 26 8 31 0.77 0.77 −0.01 17 10 36 45 6 9 9 9 −0.40 0.12 −0.52 18 <1 52 160 10 26 4 36 −0.41 0.58 −0.98 19 11 40 140 9 20 5 9 0.94 0.79 0.14 20 3 62 290 14 30 11 16 −0.01 1.13 −1.14 INVENTION 21 11 39 205 11 23 12 39 0.78 0.75 0.03 22 7 48 195 12 29 11 45 0.77 0.84 −0.07 23 11 24 85 7 15 6 40 0.34 0.80 −0.46 24 10 21 90 9 18 12 44 0.81 1.12 0.31 25 8 20 75 10 16 13 44 1.20 0.82 0.38 26 7 22 80 11 19 14 45 1.19 1.12 0.07 27 12 42 180 10 21 7 42 0.80 0.78 0.01

The invention is based essentially on the study of the actions and interactions of the elements carbon, chromium, molybdenum, vanadium and nickel and of the influence of the austenisation temperature before quenching on the mechanical properties of the steels studied.

Influence of the Austenisation Temperature:

The austenisation temperature decides the partitioning of the alloy elements between the undissolved carbides and the matrix. The dissolution of the carbides is all the more advanced as the temperature rises.

The undissolved carbides must remain in an adequate quantity on the final product in order to control the grain size. A fine grain is necessary in order to guarantee the properties of toughness and of resistance to fatigue.

The alloy elements dissolved in the matrix govern the quenchability, the resistance to annealing and in general all of the mechanical properties.

Table 2 illustrates, for one of the compositions studied (reference melt 10), the effect of the quenching temperature on the microstructure and the properties.

TABLE 2 Experimental casting reference 10: Effect of the austenisation temperature on the microstructure (distribution of the elements C and V) and the mechanical properties. Austenisation temperature 1000° C. 1030° C. 1060° C. 1075° C. At the austenisation temperature: Molar percentage 0.81% 0.52% 0.20% 0.03% of carbides VC Percentage of 0.51% 0.62% 0.75% 0.82% vanadium in the matrix Percentage of 0.26% 0.28% 0.32% 0.33% carbon in the matrix In the quenched/tempered state: Tempering 605° C. 616° C. 622° C. 624° C. temperature (duration 2 h) in order to obtain 47 HRC Energy of 38.7 J 26.9 J 16.4 J 11.6 J flexion due to shock at 20° C. for a hardness of 47 HRC (Charpy V testpiece) Loss of hardness 6.5 HRC 4.4 HRC 2.8 HRC 1.7 HRC by maintaining for 80 hours at 560° C. (initial hardness of 47 HRC)

In a context where the carbides of vanadium dissolve very progressively, the increase in the austenisation temperature causes, in this case, both an improvement in the resistance to softening when hot and a loss of toughness.

It appears that the definition of an optimal material for the envisaged applications must of necessity combine the composition and the austenisation conditions. The thermodynamic simulation, by the description of the phase equilibriums with the calculation code THERMOCALC® currently utilised by metallurgists, provides concrete elements of information on the amount of undissolved carbides for each of the types VC, M₂₃C₆ and, possibly, M₆C, Fe₃C, M₂C . . . . FIG. 1 was produced with the aid of such a simulation. It shows the evolution of the fraction of undissolved carbides according to the austenisation temperature for five reference compositions (FIGS. 1 a) to 1 e)) and a composition according to the invention (FIG. 1 f)).

The competition between the elements Mo and V for fixing the carbon according to their preferred carbide types is well established. The nickel which may be added only has a secondary effect on these mechanisms.

The experimental microstructural observations on the crude quenching state confirm the tendencies predicted by the simulation. The austenisation temperatures are optimised according to the following principles:

-   -   at a suitable temperature the carbides of types M₆C and M₂₃C₆,         not very effective for monitoring of the grain size, must be         dissolved so that the metal elements M and the carbon released         provide a maximum quenchability potential for the matrix.     -   a minimal percentage of the order of 0.20% of a molar fraction         of undissolved vanadium carbides according to the thermodynamic         estimation is necessary in order to guarantee the homogeneity         and the fineness of the grain; the austenisation temperature         must remain lower than the corresponding threshold.     -   the specified temperature should take into account a tolerance         of more or less 10 to 15 degrees with respect to this reference,         corresponding to the usual dispersion of temperature in the         charge of the industrial batches.

The austenisation temperatures of the various compositions thus defined are recapitulated in Table 3:

TABLE 3 Definition of the ideal austenisation temperatures for the various experimental melts. Casting 1, 2 3 4 5 6, 7 8, 9 10 11 12 13 14 15 Temperature 990 1005 1005 1020 1025 1030 1030 1030 1000 1000 1005 1025 (° C.) Casting 16 17 18 19 20 21, 22 23 24 25 26 27 Temperature 1025 1000 1010 1010 1030 1030 1030 1030 1030 1030 1030 (° C.)

Definition of the Optimised Compositions and Measurement of the Key Properties:

As has been said, an essential object of the invention consists of defining an equilibrium between:

-   -   on the one hand the elements molybdenum, vanadium and optionally         tungsten which favour the softening and the resistance to         softening in operation but with a weakening effect.     -   on the other hand, nickel is beneficial to the toughness but         detrimental to the hardness when hot.

Knowing that the steels in the field of the invention must exhibit a hardness when hot which is sufficient in order to avoid recessing and to resist fatigue, and that in a first approximation they exhibit the same relation between hardness at 20° C. and hardness when hot, they have been compared in quenched and tempered thermal states which give them the same hardness at 20° C. The preselected levels are 47, 45, 42 HRC.

According to an original and innovative procedure, the measurements were carried out systematically and simultaneously on laboratory test bars capable of being quenched at a high speed and on testpieces quenched in an experimental device reproducing a quenching speed representative of the treatment of industrial parts and chosen to be equal to 22° C. per minute on average within the range 900/400° C.

The measurements include:

-   -   the description of the evolution of the hardness according to         the tempering temperature for a double tempering of 2 hours in         order to define the temperings to be applied in order to achieve         the envisaged hardnesses;     -   the resistance to softening measured by the loss of hardness         caused by maintaining for 80 hours at 560° C. at an initial         state of hardness 47 HRC;     -   the toughness measured by the energy of flexion due to shock on         Charpy V testpieces broken at staggered temperatures between +20         and 200° C.

Re-Austenisation Point Ac1:

In operation this point must not be exceeded because the structural modifications of the material of the part which would result therefrom would cause a notable alteration of the mechanical properties.

According to Table 4 which shows the most representative results obtained on various samples, it is confirmed that the elements Mo and V do not have a clear influence; on the other hand, the point Ac1 is lowered the more the content of nickel is increased. Consequently the compositions with a high nickel content must be avoided for the applications where the surface temperature in operation is very high (as in the case of certain forging tools), but they remain compatible with multiple applications, such as foundry moulds for light alloys which are subjected to more moderate surface temperatures.

TABLE 4 Evolution of the re-austenisation point Ac1 according to the composition. Nickel Molybdenum Vanadium Point Ac1 Casting (%) (%) (%) (° C.)  1 0.06 1.21 0.47 825  7 0.08 2.29 0.57 820  8 0.15 1.62 0.64 805 12 1.42 1.21 0.46 770 13 2.93 1.23 0.47 680 20 0.59 2.14 0.77 800 22 (inv.) 1.63 1.82 0.71 755 23 (inv.) 1.05 1.78 0.70 785 26 (inv.) 2.19 2.28 0.70 710

Resistance to Tempering and to Softening in Operation:

Table 5 illustrates the effect of the alloy elements on the resistance to reduction in hardness whilst being kept at high temperature.

The hardnesses of 47 and 42 HRC are obtained after two temperings each of two hours, the first at 550° C., the second at the characteristic temperature appearing in the table.

The loss of hardness is measured at an initial state of 47 HRC. Table 5A shows the results obtained on a reference sample 1 and on two samples 12, 13 having a nickel content higher than the reference sample. Table 5B shows the results obtained on the sample 1 and on samples 3, 5, 6, 8 which show contents of Mo and possibly V which are higher than those of the sample 1. Table 5C shows the results obtained on samples 8 and 22 on the one hand and 6 and 26 on the other hand which show contents of Ni, Mo and V which are higher than the sample 1.

TABLE 5 Effect of the alloy elements on the recording of tempering temperature and on the softening when maintained for a prolonged period Rapid quenching Industrial quenching Loss of Loss of Temper- Temper- hardness Temper- Temper- hardness ing for ing for in 80 h ing for ing for in 80 h % Ni % Mo % V 47 RC 42 RC at 560° C. 47 HRC 42 HRC at 560° C. Casting % % % ° C. ° C. ΔHRC ° C. ° C. ΔHRC 5 - A: Effect of an addition of nickel on the reference composition 1 0.06 1.21 0.47 603 625 7.0 605 619 8.0 12  1.42 1.21 0.46 593 618 7.5 597 623 8.0 13  2.93 1.23 0.47 588 611 8.5 592 613 9.0 5 - B: Effect of additions of molybdenum and vanadium on the reference composition 1 0.06 1.21 0.47 603 625 7.0 605 619 8.0 3 0.16 1.74 0.49 605 630 5.0 608 624 6.0 5 0.17 2.74 0.48 622 648 5.0 620 637 6.5 6 0.11 2.24 0.54 617 640 5.0 617 638 6.0 8 0.15 1.62 0.64 610 638 4.5 612 631 6.5 5 - C: Effect of combined additions of nickel, molybdenum and vanadium, samples outside the invention (castings 6, 8) and samples according to the invention (castings 22, 26) 6 0.11 2.24 0.54 617 640 5.0 617 638 6.0 8 0.15 1.62 0.64 610 638 4.5 612 631 6.5 22 (inv.) 1.63 1.82 0.71 608 632 6.0 609 632 6.5 26 (inv.) 2.19 2.28 0.70 612 635 5.5 615 635 5.5

Table 5-A demonstrates the detrimental effect of a simple addition of nickel which lowers the tempering temperature too markedly for a recording of hardness and increases the loss of hardness when kept hot for a prolonged period. A lowering of the tempering temperature is damaging in that the steel must offer the highest possible operating temperature, situated at least between 600 and 630° C., for fear of softening it excessively.

As the surface temperatures of the parts are often close to 520-560° C. during injection of aluminium and even higher during forging, this criterion will be important to take into consideration in order to determine whether a given composition is or is not capable of being used for a given application.

Table 5-B shows the beneficial effect of the simple additions of molybdenum and vanadium in order to increase the resistance to tempering and to softening in operation. On the other hand, the reduction of the quenching speed between the laboratory conditions and the industrial conditions is detrimental for these characteristics, which is due to insufficient quenchability of the material.

The comparison of the composition pairs (8, 22) and (6, 26) in Table 5-C illustrates that in laboratory conditions the castings with nickel offer a lesser resistance to the lowering of the hardness than the corresponding castings with a low nickel content, but that with industrial quenching their properties become very close.

To summarise, under the conditions of an industrial thermal treatment the combined and balanced addition of nickel, molybdenum and vanadium confers properties of resistance to tempering and to softening by maintaining for prolonged periods which are equivalent to the properties of the grades without nickel.

These favourable results are explained by the significant increase in quenchability which is illustrated in the appended FIG. 2 which compares the QCC continuous cooling diagrams of the reference composition 1 (FIG. 2 a) which have been subjected to an austenisation at 990° C. for 30 minutes and of the composition 22 according to the invention (FIG. 2 b) which has been subjected to an austenisation at 1030° C. for 30 minutes.

The composition according to the invention has pearlitic zones and bainitic zones which are clearly offset towards the low cooling speeds relative to the reference composition. Consequently, knowing that the usual industrial quenchings (of which the paths are shown in bold in FIGS. 2 a and 2 b) make it possible to reach, on the tools to be treated, a temperature of 400° C. in 1000 to 5000 seconds according to the sizes of the parts and the situation in the part, the composition according to the invention enables an exclusive martensitic transformation. On the contrary, the reference composition necessitates the formation of a significant proportion of bainite, which is less favourable to obtaining the envisaged properties.

Toughness:

The unfavourable effect of the reduction in the quenching speed between the laboratory conditions and the industrial conditions appears even more accentuated on the breaking energy of Charpy V testpieces of flexion due to shock.

Table 6 illustrates the representative trends over a selection of results; the combined addition of Ni, Mo, V effected on the casting 21 according to the invention is favourable at the same time for obtaining the highest resilience values after treatment under the industrial conditions and the slightest reduction caused by the slowing down of the quenching speed.

TABLE 6 Energy of breaking due to shock on the periphery of Charpy V testpieces measured for several representative castings with: Breaking energy of Charpy V testpieces (Joules) Hardness 45 HRC Hardness 47 HRC at 20° C. at 100° C. at 20° C. at 100° C. Casting Ni % Mo % V % R L R L R L R L 1 0.06 1.21 0.47 41.0 26.5 59.0 33.5 31.0 20.5 53.5 29.0 5 0.17 2.74 0.48 19.0 12.0 35.0 17.5 18.0 11.0 27.0 13.0 6 0.11 2.24 0.54 24.0 16.0 50.0 28.0 21.0 15.0 37.0 20.0 8 0.15 1.62 0.64 31.0 20.0 49.5 26.0 26.0 17.0 42.0 22.0 20  0.59 2.14 0.77 25.5 17.5 50.5 30.5 23.0 16.0 38.5 19.5 21 (inv.) 1.64 1.76 0.67 40.5 31.0 65.5 54.0 31.5 24.5 54.5 40.0

R: rapid quenching speed (oil quenching of the bar)

L: slow speed (industrial speed reproduced in the laboratory).

The appended FIG. 3 compares, for all of the castings, the values obtained with a quenching according to the industrial speed and those resulting from a rapid quenching for one and the same composition of the metal, wherein the pairs of batches of testpieces then undergo annealings in order to record hardnesses of 42, 45 and 47 HRC and the testpieces are broken at 20° C. and at 100° C. Each point is representative of a hardness and of a temperature of breaking of the testpiece. The results demonstrate that the loss of hardness due to the reduction in the quenching speed is very generally more limited for the compositions according to the invention.

The trends expressed by the laboratory tests are confirmed by tests on tooling blocks treated according to the following conditions:

-   -   Blocks of dimension 570×450×228 mm     -   Identical positioning in the furnace     -   Quenching in the same industrial furnace under gas pressure of 5         bars, with the same gas flow     -   Double tempering with individual adjustment of the temperatures         in order to obtain the level of hardness of 46+/−0.5 HRC.

Sampling of Charpy V testpieces of flexion due to shock in the transverse direction: in the centre of the large face close to the skin and to the core of the block.

The average values of the energies of flexion due to shock set out in Table 7 confirm that the steel 22 according to the invention has superior properties, in particular in the core block position, a position which represents even larger part sizes.

TABLE 7 Result of tests of flexion due to shock on tool blocks treated in industrial conditions Breaking energy at Breaking the energy at periphery the core Casting Ni % Mo % V % KV (Joules) KV (Joules) 2 0.11 1.27 0.49 32 16 5 0.17 2.74 0.48 18 14 7 0.08 2.29 0.57 23 20 9 0.25 1.59 0.66 24 19 22 (inv.) 1.63 1.82 0.71 28 26

All of these results of mechanical tests illustrate the detrimental effects of the lowering of the quenching speed, in particular:

-   -   the lowering of the energy of flexion by shock at equal hardness     -   the increase in the loss of hardness by maintenance for a         prolonged period at 560° C.

Nevertheless, the amplitude of these alterations is not identical for all the compositions, and it is verified that the simultaneous and balanced addition of the alloy elements according to the rules specified below significantly reduces it.

Effects of the Alloy Elements:

It has been possible to evaluate the effects of the various alloy elements and their interactions by the comparative experimental study of the properties of the experimental castings and to interpret them by thermodynamic simulation. By following the principles set out above concerning the quenching conditions, the following trends were confirmed

Carbon favours the quenchability, increases the ideal austenisation temperature and determines the maximum hardness obtained after annealing to 550° C. However, it has a detrimental effect on the toughness. Associated with high contents of molybdenum or vanadium, it can lead to the formation of eutectic carbides which are detrimental to the microstructure and to the toughness. Their level should be within the range situated between a value of at least 0.30% necessary for obtaining a sufficient hardness and of 0.39% at most in order to avoid an irremediable fragility. The optimum range is from 0.33 to 0.38%.

Chromium has a favourable effect for the quenchability. It plays a part in the hardening by tempering, and for the preferred applications envisaged by the invention, namely large parts which necessitate a high hardness (42 to 52 HRC), this characteristic is advantageous. However, the carbides which it generates evolve quickly to more stable forms and do not prove very effective for the resistance to the reduction in hardness at high temperature. It is therefore essential to complement the addition of Cr by other carbide-forming elements such as Mo and V. The content of this element must remain limited between a minimum of 4.0% necessary for the quenchability and a maximum of 6.0% above which its action partially inhibits that of vanadium and of molybdenum. Preferably, a Cr content of 4.6 to 6% is set.

Molybdenum improves the quenchability. It combines with chromium in the same chromium-based carbides, which contributes to an increase in the number thereof. At high contents it forms specific species M₂C, M₆C. With regard to the macroscopic properties it increases the hardness and the resistance to tempering and decreases the toughness. Its content is between 1.50 and 2.60%. It is also necessary to take into account the possible presence of tungsten as will be described below. Preferably, the Mo content is between 1.60 and 2.00% with Mo+0.65W between 1.60 and 2.20%.

Vanadium forms specific carbides of the VC type which, in the area covered by the experimental castings, are predominant among the precipitates which are not dissolved at the austenisation temperature and thus ensure that the grain does not enlarge. In the course of the tempering carried out after quenching, new generations of micro- and nanometric carbides are precipitated and by their interaction with crystal defects of the martensite participate actively in the secondary softening and in the resistance to softening in operation under the effect of the temperature and of the cyclical forces. On the other hand, an excess of these carbides formed during tempering causes a marked weakening. Within the context of the compositions studied, and following the principles decreed for the choice of the austenisation temperature, the vanadium content must of necessity be between 0.55% and 0.75%.

Nickel has a negative effect on the hardness in the treated state; it decreases the tempering temperature to be applied in order to obtain an envisaged hardness, and the resistance to softening while maintained at operating temperatures. Moreover an excessive content of the order of 3% lowers the re-austenisation point too markedly within the range of the temperatures used, which must be absolutely avoided. On the other hand, nickel increases the quenchability, in particular for contents of 1 to 3% and significantly improves the toughness. It is considered that within the context of the invention the Ni content is between 0.80 and 2.80%. The negative effects on the hardness of a substantial addition of Ni can be compensated for by additions of Cr, Mo, V and W within the prescribed limits.

Tungsten may constitute an optional additional element within the limit of 1.45% maximum and under conditions such that the content of Mo+0.65W is between 1.50 and 3.20% with the Mo content between 1.50 and 2.60%, preferably between 1.60 and 2.20% with the Mo content between 1.60 and 2.00%. In effect, the tungsten complements the action of the molybdenum with an equivalence ratio of 1% for 0.65% of Mo. This addition of tungsten causes limited negative effects on the toughness and the quenchability and positive effects on the resistance to softening when hot, in particular for test temperatures higher than 560° C., for example 600° C.

Cobalt may be added up to an upper limit of 2.75%. It has a favourable effect for the resistance to softening, in particular for residence temperatures of the order of 600° C., but its action is detrimental to the quenchability. Taking account of the high price of this additional element, it does not appear that its use must be particularly recommended.

Moreover, obtaining an ideal compromise between properties in use demands that the simultaneous additions of molybdenum, vanadium, nickel and possibly tungsten are balanced and satisfy the following relations:

K between −0.65 and +0.65, preferably between −0.35 and +0.35, optimally as close to zero as possible, with:

K=K2−K1

K2=0.75×(% Ni−0.60)

K1=1.43×(% V−0.40)+0.63×[% Mo+(0.65×% W)−1.20]

It has been seen that Table 1 sets out the values of the coefficients K1, K2, K for all of the castings.

The best results are obtained when the following conditions occur simultaneously:

0.335%≦C≦0.375%;

and 1.50%≦Ni≦2.10%;

and 1.60%≦Mo+0.65W≦2.00%, with Mo≧1.60%;

and 0.62%≦V≦0.75%.

For more specific applications the following simultaneous conditions may also be recommended:

-   -   0.335%≦C≦0.375%

and 2.00%≦Ni≦2.40%

and 1.80%≦Mo+0.65W≦2.90% with 1.80≦Mo≦2.40% and W≦0.90%

and 0.66%≦V≦0.76%

when it is desired to obtain a remarkable quenchability for the manufacture of large parts reserved for applications for which, taking account of the lowering of the point of transformation A1 by Ni, the working temperature at the surface remains below 680° C., for example the applications of extrusion dies or moulds for founding alloys of Al;

-   -   0.335%≦C≦0.375%

and 0.90%≦Ni≦1.50%

and 1.50%≦Mo+0.65 W≦1.90% with W≦0.40%

and 0.55%≦V≦0.63%

when properties are required which are remarkable for medium-sized parts and are suitable for applications for which the surface temperature in operation remains below 770° C.

Moreover, other elements which will be mentioned must or can be present within precise limits.

Silicon, due to its detrimental effect on the toughness, must be kept at a low level compatible with economic industrial production conditions; a limit of 0.50% and preferably of 0.40% must not be exceeded.

Manganese, which is favourable to the quenchability, but detrimental to the toughness, must not be present in a content higher than 0.80%, preferably 0.60%.

The elements sulphur, phosphorus, arsenic, tin, antimony, titanium, zirconium, niobium, nitrogen are unfavourable to the toughness and capable of inducing a weakening in operation and must be limited to the lowest contents compatible with industrial and economic constraints. The maximum permissible contents are:

-   -   for S: 0.0040%, preferably 0.0010%     -   for P: 0.0200%, preferably 0.0080%     -   for Ti: 0.05%, preferably 0.01%     -   for Zr: 0.05%, preferably 0.02%     -   for Nb: 0.08%, preferably 0.01%     -   for N: 0.0400%, preferably 0.0100%

Furthermore, the contents of P, As, Sb, Sn must satisfy the following relation:

10P+As+5Sb+4Sn≦0.21%, preferably ≦0.10%.

The aluminium content must be between traces and 0.080%, preferably between traces and 0.030%. Its function is to deoxidise the steel, thus limiting the quantity of inclusions of oxides capable in particular of decreasing the resistance to fatigue of the steel. From this point of view and simultaneously the oxygen content must not exceed 30 ppm, preferably 15 ppm. A high Al content reduces the O content dissolved in the liquid steel, but it also renders the liquid steel more sensitive to atmospheric reoxidations during casting and therefore increases the risk of forming detrimental oxidised inclusions.

In a general manner the steels according to the invention can fall within two quality levels.

A “standard” level of quality is attained when the composition does not absolutely have to be situated within the optimal ranges which are defined above for all the elements. The improvement relative to the prior art then resides above all in the properties of quenchability. These allow the manufacture of large products which have a high hardness and are homogeneous in the entire section of the products.

A “superior” level of quality is attained when all the elements are situated within the optimal ranges of contents defined above. Under these conditions, in addition to the improved quenchability, a high toughness is obtained which provides, in conjunction with the high hardness, a great resistance to thermal fatigue and to sudden rupture.

In order to obtain such results, it is necessary to resort to a mode of production which includes, after primary refinement in an electric furnace and in a ladle, refusion of the consumable electrode by processes of vacuum arc refusion (VAR) or of electroconductive slag refusion (ESR), which in particular make possible the very low O contents envisaged. Equally, as is usual on these types of steel, it is necessary to provide on the cast steel a thermomechanical process of rolling and of annealing which gives the steel a compact, coalesced, fine and homogeneous structure, in conjunction with solidification conditions which generate dendrites which are small and not very segregated.

Amongst the parts which can be manufactured from the steel according to the invention produced as has been described are included in particular the parts for tooling for hot shaping in general, and particularly,

-   -   moulds or dies for founding under pressure of light alloys or of         cuprous alloys;     -   forging dies;     -   tools for drilling and for rolling of steel tubes;     -   tools for shaping glass and plastics materials.

The invention has a preferred application in the manufacture of such parts having a thickness of 200 mm and more. 

1. Steel for hot tooling, having a composition in percentages by weight: 0.30%≦C≦0.39% 4.00%≦Cr≦6.00% traces≦Si≦0.50% traces≦Mn≦0.80% traces≦W≦1.45% traces≦Co≦2.75% 0.80%≦Ni≦2.80% 1.50%≦Mo≦2.60% with 1.50%≦Mo+0.65W≦3.20% 0.55%≦V≦0.80% with −0.65≦K≦0.65 where K=K2−K1 and K2=0.75×(Ni−0.60) K1=1.43×(V−0.40)+0.63×[(Mo+0.65W)−1.20] traces≦Al≦0.080% traces≦S≦0.0040% traces≦P≦0.0200% traces≦Ti≦0.05% traces≦Zr≦0.05% traces≦Nb≦0.08% traces≦N≦0.040% 10P+As+5Sb+4Sn≦0.21% traces≦O≦30 ppm the rest being iron and inevitable impurities.
 2. Steel according to claim 1, characterised in that 0.33%≦C≦0.38%.
 3. Steel according to claim 1, characterised in that traces≦Si≦0.40%.
 4. Steel according to claim 1, characterised in that traces≦Mn≦0.60%.
 5. Steel according to claim 1, characterised in that 4.6%≦Cr≦6.0%.
 6. Steel according to claim 1, characterised in that 1.60%≦Mo≦2.00% and 1.60%≦Mo+0.65 W≦2.20%.
 7. Steel according to claim 1, characterised in that traces≦Al≦0.030%.
 8. Steel according to claim 1, characterised in that traces≦S≦0.0010%.
 9. Steel according to claim 1, characterised in that traces≦P≦0.0080%.
 10. Steel according to claim 1, characterised in that traces≦Ti≦0.01%.
 11. Steel according to claim 1, characterised in that traces≦Zr≦0.02%.
 12. Steel according to claim 1, characterised in that traces≦Nb≦0.01%.
 13. Steel according to claim 1, characterised in that traces≦N≦0.01%.
 14. Steel according to claim 1, characterised in that 10P+As+5Sb+4Sn≦0.10%.
 15. Steel according to claim 1, characterised in that traces≦O≦15 ppm.
 16. Steel according to claim 1, characterised in that −0.35≦K≦0.35.
 17. Steel according to claim 1, characterised in that 0.335%≦C≦0.375% 1.50%≦Ni≦2.10% 1.60%≦Mo+0.65 W≦2.20% with 1.60%≦Mo≦2.00% 0.62%≦V≦0.75%.
 18. Steel according to claim 1, characterised in that 0.335%≦C≦0.375% 2.00%≦Ni≦2.40% 1.80%≦Mo+0.65W≦2.90% with 1.80%≦Mo≦3.40% and W≦0.90% 0.66%≦V≦0.76%.
 19. Steel according to claim 1, characterised in that 0.335%≦C≦0.375% 0.90%≦Ni≦1.50% 1.50%≦Mo+0.6W≦1.90% with W≦0.40% 0.55%≦V≦0.63%.
 20. Steel according to claim 1, characterised in that: 0.335%≦C≦0.375%, 4.60%≦Cr≦6.00%, traces≦Si≦0.40%, traces≦Mn≦0.60%, traces≦W≦1.45%, traces≦Co≦2.75%, 1.50%≦Ni≦2.10%, 1.60%≦Mo+0.65 W≦2.20% with 1.60%≦Mo≦2.00%, 0.62%≦V≦0.75%, with −0.35≦K≦0.35, traces≦Al≦0.030%, traces≦S≦0.0010%, traces≦P≦0.0080%, traces≦Ti≦0.011%, traces≦Zr≦0.02%, traces≦Nb≦0.01%, traces≦N≦0.01%, traces≦O≦15 ppm.
 21. Method of manufacture of a steel part, characterised in that the said part is prepared from a steel according to claim 1, and in that it is subjected to an austenisation within the temperature range from 1000 to 1050° C., followed by quenching.
 22. Method according to claim 21, characterised in that the austenisation takes place in the range from 1015 to 1040° C.
 23. Method according to claim 21, characterised in that after the quenching the part is subjected to at least two temperings within the temperature range from 550 to 650° C., giving the said part a hardness of 42 to 52 HRC.
 24. Part made from steel obtained by the method according to claim 21, characterised in that it is a part for tooling for hot shaping.
 25. Part according to claim 24, characterised in that the said part has a thickness greater than or equal to 200 mm.
 26. Part according to claim 24, characterised in that it is a mould or a die for founding under pressure light alloys or cuprous alloys.
 27. Part according to claim 24, characterised in that it is a forging tool.
 28. Part according to claim 24, characterised in that it is a forging die.
 29. Part according to claim 24, characterised in that it is a tool for drilling or for rolling of steel tubes.
 30. Part according to claim 24, characterised in that it is a tool for shaping glass.
 31. Part according to claim 24, characterised in that it is a tool for shaping plastics materials.
 32. Part according to claim 24, characterised in that it is produced from a steel wherein 0.335%≦C≦0.375% 2.00%≦Ni≦2.40% 1.80%≦Mo+0.65W≦2.90% with 1.80%≦Mo≦3.40% and W≦0.90% 0.66%≦V≦0.76%; and in that it is an extrusion die or a mould for founding aluminium alloy.
 33. Use of a part according to claim 24, characterised in that the said part is made from a steel wherein 0.335%≦C≦0.375% 2.00%≦Ni≦2.40% 1.80%≦Mo+0.65W≦2.90% with 1.80%≦M≦3.40% and W≦0.90% 0.66%≦V≦0.76%; and in that its working temperature at the surface remains below 680° C.
 34. Use of a part according to claim 24, characterised in that the said part is made from a steel wherein 0.335%≦C≦0.375% 0.90%≦Ni≦1.50% 1.50%≦Mo+0.6W≦1.90% with W≦0.40% 0.55%≦V≦0.63%; and in that its surface temperature in operation remains below 770° C. 